Transmission electron microscopy investigation of fatigue crack tip plastic zones in a polycrystalline gamma-TiAl-based alloy

Deformation at the tips of fatigue cracks has been studied in a polycrystalline, fully lamellar TiAl based alloy using transmission electron microscopy. A tiny, intensely deformed plastic zone containing twins, slip bonds and microcracks is generated during fatigue crack growth. Twins are activated extensively, for a wide range of Schmid factors. When the Schmid factor for slip is large, the resulting slip band is compact and well defined. Such slip bands tend to be associated with microcracks. When the Schmid factor for slip is small, the resulting slip band is diffuse and ill defined. Such slip bands are not associated with microcracks. Microcracks form, not only along well defined slip bands, but also along alpha (2)-gamma interfaces and, less willingly, along gamma-gamma twin interfaces. The former, however, present more resistance to translamellar deformation than do the latter. Deflection of microcracks from interlamellar and twin mode to slip band mode and vice versa, and the transfer of translamellar deformation from lamella to lamella, can be understood by reference to the competing effects of the externally applied stress (via the Schmid factor) and neighbouring pile-ups and microcracks.